<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        let year = +prompt('请输入年份')
        if (isNaN(year)) {
            alert('输入不合法，请输入年份')
        } else if (year % 100 === 0) {
            if (year % 400 === 0) {
                document.write(year + '是闰年')
            } else {
                document.write(year + '不是闰年')
            }

        } else if (year % 4 === 0) {
            document.write(year + '是闰年')
        } else {
            document.write(year + '不是闰年')
        }
    </script>
</body>

</html>